CHAPTER 19 NOTES
FOSSILS AND ANCIENT LIFE
Fossil Record: The fossil record provides evidence about the history of life that existed on Earth.
It also shows how various organisms including species, have changed over time.
99% of all species that lived on Earth are now extinct.
Extinct species is one that has dies out.
How do you think these extinct organisms lived? Where do you think they lived? What kinds of organisms to you think they are related to that exist today?
Dinosaur Dig for T-Rex
Types of Fossils:
A fossil can be either partial or complete remains of an organism.
How Fossils Form:
Sedimentary rock must be present.
1. Exposure to heat, rain, wind, or cold breaks down existing rock into small particles of sand, silt, and clay.
2. These small particles are carried by streams and rivers into lakes or seas, where they eventually
settle to the bottom.
3. Layers of sediment build up gradually over time.
4. If dead organisms (animals, plants, protists, bacteria, or fungus) become buried in the sediment quickly then a pocket will form with no oxygen. Bacteria can’t survive and there is no decay.
5. The weight of the layers of sediment gradually compress the lower layers into rock.
6. The preserved remains may later become exposed.
What Fossils Can Reveal:
Paleontologists: researchers that study fossils to learn about ancient life.
1. Anatomical similarities and differences between the fossil organism and organisms living today.
2. The fossil’s age.
3. What the environment may have been like when the organism was alive. This may tell you what the organism ate and how it lived.
Techniques for determining the age of a fossil:
1. Relative Dating: The age of the fossil is determined by comparing its placement in the sedimentary rock with that of fossils in other layers.
Relative dating only tells you an estimate of the age (whether one fossil is older or younger than another fossil).
Older fossils are found in the lower layers of sedimentary rock.
§ Fossils of a species that are easily recognized.
§ Existed for only a short period of time so will only be found in a few layers of rock.
§ Had a wide geographic distribution.
2. Radioactive Dating:
Scientists calculate absolute ages of the fossil based on the proportion of radioactive isotope to stable isotope it contains.
Example: Carbon-14: decays into nitrogen-14 every 5739 years which is called the half-life. Carbon-12 does not decay.
Example: Potassium-40: decays into argon-40 every 1.26 billion years.